Immunological memory is a cardinal property of the adaptive immune system and memory CD8 T cells play a critical role in providing long-term protective immunity against reoccurring infectious disease and cancers. Generating long-lived functional memory T cells is vital for any successful vaccination campaign. For the greatest protection possible memory T cells must possess 1) longevity, 2) high proliferative potential and 3) rapid development of effector functions when antigen is reencountered. It is not known how these properties are instilled into the precursors of memory T cells during immune responses, and this is the primary focus of this proposal. Our prior studies using acute viral and bacterial infection in mice embarked on identification of the memory CD8 T cell precursors, and this work helped to distinguish this subset of effector cells from those that will die. Our recent preliminary results outline formation of memory precursor effector cell (MPEC) and short-lived effector cell (SLEC) lineages during infection and delineate when effector CD8 T cells begin to adopt the SLEC fate. We found that the transcription factor T-bet is a key lineage-determining factor that controls the SLEC/MPEC fate decision during infection, and that the relative concentration of T-bet in the effector CD8 T cells modulated this process. High levels of T-bet specified SLEC development whereas low levels specified MPEC development. Furthermore, we found that IL-12 was a potent inducer of T-bet expression and SLECs during infection. Thus, early inflammatory signals not only define the types of effector CD8 T cells that form and enhance their functions during infection, but they also influence their longevity and ability to develop into memory CD8 T cells that can self-renew. This highlights a novel control mechanism by which the innate immune system regulates T cell homeostasis. The over-arching goal of this proposal is to elucidate the mechanisms that determine which effector CD8 T cells become MPECs or SLECs during immune responses in order to create new avenues for improving vaccination and immunotherapies that fight infectious disease and cancer. Specifically, we aim to (i) determine when T-bet functions during effector CD8 T cell differentiation and how T-bet and IL-12 signals are coordinately integrated, (ii) identify other cytokines that regulate T-bet expression and terminal effector cell differentiation, (iii) determine if T-bet controls the memory cell potential of Th1 effector CD4 T cells during LCMV infection.